The invention relates to microcapsules whose capsule walls comprise a resin which is obtainable by reacting at least one amine and/or defined aromatics or heteroaromatics and at least one aldehydic component which has at least two C atoms per molecule in the presence of at least one copolymer which comprises units of AMPS and/or AMPP and (meth)acrylates, and also to dispersions comprising such microcapsules. Additionally provided by the invention are the use and the production of the microcapsules/microcapsule dispersions, and also products which comprise such microcapsules/microcapsule dispersions, and the use thereof.
From the prior art, microcapsules are known which may comprise liquid, solid or gaseous substances as core material. Commonplace materials for the capsule walls are, for example, phenol-formaldehyde polymers, melamine-formaldehyde polymers, polyurethane, gelatin, polyamides or polyureas. Widespread, for example, is the use of microcapsules filled with leuco dye for the production of carbonless copy papers.
From U.S. Pat. No. 3,755,190 it is known that capsules made of phenol-formaldehyde polymers have fragile walls. In order to avoid this, a production method is described in which fully hydrolyzed polyvinyl alcohol is used.
Dispersions of microcapsules made from aminoplast resins, such as melamine-formaldehyde resins, inevitably contain—as a result of their production—a certain fraction of free formaldehyde. For environmental and workplace safety reasons it is desirable to minimize the formaldehyde content, and if possible to avoid formaldehyde entirely.
For the purpose of reducing the formaldehyde content it is customary to add formaldehyde scavengers to microcapsule dispersions based on melamine-formaldehyde resins. The formaldehyde scavengers most frequently used include ammonia, urea, ethyleneurea, and melamine, which reduce the residual formaldehyde content of the capsule dispersion.
EP-A 0 383 358 and DE-A 38 14 250 disclose light-sensitive materials consisting of microcapsules whose walls are formed from melamine-formaldehyde resins. Urea is added in the course of curing, for the purpose of removing excess formaldehyde.
In the case of the methods described in EP-A 319 337 and U.S. Pat. No. 4,918,317, urea is added toward the end of curing.
EP-A 0 415 273 describes the production and use of monodisperse and polydisperse solid-sphere particles made of melamine-formaldehyde condensate. To bind the formaldehyde released during the condensation, the use of ammonia, urea or ethyleneurea is proposed.
Microcapsules made of melamine-formaldehyde resins produced using polymers containing sulfonic acid groups are notable for their uniform capsule size and imperviousness (EP-A 0 218 887 and EP-A 0 026 914). These capsule dispersions, however, still contain residual free formaldehyde, whose presence is unwanted during further processing.
EP-A 0 026 914 therefore recommends binding the formaldehyde, following curing, using ethyleneurea and/or melamine as formaldehyde scavengers.
Known from DE 198 35 114 are dispersions of microcapsules based on melamine-formaldehyde resin, the melamine-formaldehyde resin being partially etherified and comprising a water-soluble primary, secondary or tertiary amine or ammonia. Prior to curing, urea is added as a formaldehyde scavenger.
DE 198 33 347 describes a method for producing microcapsules by condensation of melamine-formaldehyde resins and/or their methyl ethers, where urea or urea whose amino groups are joined with an ethylene or propylene bridge is added as a formaldehyde scavenger prior to curing. The resulting dispersions are indeed low in formaldehyde, but the addition of urea prior to curing adversely affects the stability of the microcapsules and the viscosity of the microcapsule dispersion.
WO 01/51197 teaches a method for producing microcapsules by condensation of melamine-formaldehyde resins, where a mixture of melamine and urea is added in the course of curing.
The addition of the stated formaldehyde scavengers to the completed microcapsule dispersion or during the production of the microcapsule dispersion generally lowers the formaldehyde content of the dispersion. Often, however, it is not possible to reduce below a defined limit the formaldehyde content of products which comprise such microcapsule dispersions or have been treated with such microcapsule dispersions, even when large amounts of formaldehyde scavengers are added.